1. Technical Field
This disclosure relates to a polarimeter. In particular, the polarimeter may be used to measure the chirality of molecular samples through the measurement of the optical rotation of plane polarized light or circular dichroism.
2. Background Art
Detecting and measuring chirality has applications in a diverse array of fields such as the pharmaceutical, cosmetics, and food industries as well as the more general fields of medicinal and biological chemistry, chemical synthesis and analysis of microsamples, proteomics, and ecology, surface catalysis, nucleation, and cell membrane reactions, to name a few.
Conventional polarimeters, which measure an angle of rotation caused by passing polarized light through an optically active material, work by passing light through a material sample and using typical configurations, the light makes only a single pass through the sample. While useful for the typical purposes of polarimeters, the structure which permits the single pass of light through the sample also limits the potential usefulness of the polarimeter.
Gas-phase chirality measurements, using a bowtie cavity and continuous-wave (cw) lasers and measurement, are described in the article by L. Bougas, et al., Cavity-Enhanced Parity-Nonconserving Optical Rotation in Metastable Xe and Hg, Physical Review Letters, 25 May 2012, vol. 108, 210801 (2012), the disclosure of which is incorporated herein by reference. The gas-phase measurements, however, do not address how to accomplish liquid-phase and interfacial determination of chirality, or how to measure chirality using a pulsed laser
The measurement of chirality of materials is most commonly measured optically using circular dichroism (CD), the preferential absorption of right compared to left circularly polarized light, or optical rotatory dispersion (ORD), the rotation of the plane of linearly polarized light as it is transmitted through a sample. Experimental signals from both CD and ORD are usually very small. For example, the CD asymmetry parameter g is typically of order 10−4, so that nearly optically thick chiral samples demonstrate differences in absorption between right and left circularly polarized light of order 10−4. The ORD angle is proportional to optical pathlength and the sample concentration. Typically, commercial polarimeters for ORD measurements use cells with pathlengths of 1-10 cm, so that the ORD angle can be large enough to be measured accurately which require a large sample sizes most likely on the order of many milliliters of a substance. The optical principles behind measuring CD and ORD have remained largely unchanged for many decades.
The measurement of CD or ORD in an evanescent wave at the total internal reflection (TIR) of a light beam at the surface of a prism can give experimental signals of order g (i.e. CD signal differences of about 10−4, or ORD angles of about 10−4 rad), where g is the chiral asymmetry in the refractive index of right and left circularly polarized light given by n±=n±g (where n+ and n− are the refractive indices of a chiral medium for right and left circularly polarized light, respectively). Silverman and Lekner showed that close to the TIR critical angle, the chiral signals are enhanced. This is shown by the following equation (derived following the treatment of Lekner) for the optical rotation angle ΘEW following TIR, from the placement of a chiral sample in the evanescent wave of the prism:
                              Θ          EW                ≅                                            Δ              ⁢                                                          ⁢              n                        n                    ⁢                      N                          1              -                              N                2                                              ⁢                                    cos              ⁢                                                          ⁢              θ                                                                                            sin                    2                                    ⁢                  θ                                -                                  N                  2                                                                                        (        1        )            
where θ is the incidence angle, Δn=(n+−n−), n=(n++n−)/2, n+ and n− are the refractive indices of the chiral sample for left and right circularly polarized light, respectively, N=(n/np), and np is the refractive index of the prism. The ΘEW increases sharply near critical angle (sin θ≈N), and also near index matching (N≈1).
In the last 15 years, Poirson and Vaccaro pioneered the use of optical cavities to increase the path length to measure ORD of chiral vapors, by introducing two quarter wave plates in and at either ends of a two-mirror cavity. The pair of quarter wave plates carries out two functions: 1) for each pass, the light polarization is reflected through the optical axis of the wave plate so that the optical rotation of the forward and backward light path sums instead of cancelling, and 2) the optical axes of the two waveplates are offset by angle α, so that the light polarization is rotated by 2α for each roundtrip pass. Vaccaro showed that if 2α>>η (where η is the linear birefringence in the cavity, which normally causes great problems in measuring the small optical rotation of the sample), then the negative effects of the linear birefringence become negligible. However, this cavity-based technique has only been applied to vapor samples. The main problem of these linear cavities is that the signal of the chiral sample needs to be compared to the signal of a null sample. As it is difficult to alternate reproducibly between the sample and the null, and significant time is needed to do so (typically many seconds), the background subtraction of the null signal does not typically allow the measurement of small signals (e.g. smaller than about 0.1 mdeg/pass).
Very recently, Bougas et al. described a bowtie cavity with counter-propagating laser beams and an intercavity magneto-optical crystal using an applied longitudinal magnetic field. This crystal caused an optical rotation of the polarization with the same advantages as that caused by the quarter wave plates described above. However, in addition, Bougas et al. showed that the sample optical rotation angle can be reversed in sign by inverting the sign of the magnetic field. This gives an experimental control for isolating small experimental signals, without needing to remove the sample and replace with a null sample. This magnetic field reversal can be used as an efficient and rapid subtraction procedure to isolate the small chiral signal from much larger backgrounds. Bougas et al. described an experimental setup that allowed measurements using continuous wave (cw) lasers and measurement methods, on low-loss gas-phase samples.
To reduce the length and complexity of the detailed description and to establish a current state of the art, Applicant hereby incorporates by reference in their entirety each reference listed in the numbered paragraphs below:    M. P. Silverman, J. Badoz, “Large enhancement of chiral asymmetry in light reflection near critical angle” Optics communications 74, 129 (1989).    J. Lekner, “Optical properties of isotropic chiral media” Pure Appl. Opt. 5, 417 (1996).    J. Poirson, M. Vallet, F. Bretenaker, A. L. Floch, and J. Thepot, Anal. Chem. 70, 4636 (1998).    T. Müller, K. B. Wiberg, P. H. Vaccaro, J. Phys. Chem. A, 104, 5959 (2000).    J. L. Hall, J. Ye, and L.-S. Ma, Phys. Rev. A 62, 013815 (2000).    G. Bailly, R. Thon, and C. Robilliard, Rev. Sci. Instrum. 81, 033105 (2010).    V. M. Baev, T. Latz, P. E. Toschek, Appl. Phys. B 69, 171-202 (1999).    W. von Klitzing, R. Long, V. S. Ilchenko, J. Hare, V. Lefevre-Seguin, Optics Letters 26, 166-8 (2001).